I'm not a doctor, and I don't play one on T.V., but people abuse themselves in many ways and, heart wise, usually seem to pull through without complications. I have a few NIH links to people that tried to terminate themselves by injecting mercury, they were dental assistants, a few were successful, yet others, after recovering from the initial trauma, were followed up, even a few years later, and were OK. The doctor that authored the story included XRay images of the mercury distributed in the lungs, injection site, hips, etc. www.ncbi.nlm.nih.gov/pmc/articles/PMC2890926/
I realize this has little to do with your situation, save that exposures seem to be more likely to affect metabolizing organs, such as kidneys or liver, than the heart.
If you've done research and not discovered a particular reason it would effect the heart, it's not likely it has, especially in light of the examinations and testing you've had.
Beta Blockers are excellent in reducing heart rate and force of contraction. Last December, doing an Echo Stress Test, I lasted over 5 minutes, had a heart rate in the 130's (Target rate is usually 220-age), i should have made it to 165, but the legs weren't going to last, plus I was on 12.5 mg of Atenolol twice a day, but withheld the medication that morning. This August, I had a stress only test, lasted 6 minutes, but I'm on 200 mg Acebutolol three times a day and took it that morning. My heart rate after 6 minutes was 91. That's an incredible reduction.
The Lyme is something to pursue, the reaction to the herbal substances may have been coincidental to your current symptoms. I've seen some incredible coincidences over the years.
I started to write this earlier in this post, but your heart is innervated by the parasympathetic and sympathetic nervous systems. Fight or flight reaction stimulates the sympathetic system, and the parasympathetic system is suppressed, things like digestion would be slowed. When the fear is over, the parasympathetic system slows your heart and speeds digestion. Over activation of the sympathetic system makes your heart rate higher and the force of contraction harder. Yet underactivation of the parasympathetic system can, apparently, have the same effect. From what I read, the heart's natural pacemaker would run at a rate over 100 Beats per minute, but the parasympathetic system holds the rate down by directly acting on the natural pacemaker.
How's your breathing and digestion? Normal? Just curious,